drm/bridge: Extend bridge API to disable connector creation

Most bridge drivers create a DRM connector to model the connector at the
output of the bridge. This model is historical and has worked pretty
well so far, but causes several issues:

- It prevents supporting more complex display pipelines where DRM
connector operations are split over multiple components. For instance a
pipeline with a bridge connected to the DDC signals to read EDID data,
and another one connected to the HPD signal to detect connection and
disconnection, will not be possible to support through this model.

- It requires every bridge driver to implement similar connector
handling code, resulting in code duplication.

- It assumes that a bridge will either be wired to a connector or to
another bridge, but doesn't support bridges that can be used in both
positions very well (although there is some ad-hoc support for this in
the analogix_dp bridge driver).

In order to solve these issues, ownership of the connector should be
moved to the display controller driver (where it can be implemented
using helpers provided by the core).

Extend the bridge API to allow disabling connector creation in bridge
drivers as a first step towards the new model. The new flags argument to
the bridge .attach() operation allows instructing the bridge driver to
skip creating a connector. Unconditionally set the new flags argument to
0 for now to keep the existing behaviour, and modify all existing bridge
drivers to return an error when connector creation is not requested as
they don't support this feature yet.

The change is based on the following semantic patch, with manual review
and edits.

@ rule1 @
identifier funcs;
identifier fn;
@@
 struct drm_bridge_funcs funcs = {
 	...,
 	.attach = fn
 };

@ depends on rule1 @
identifier rule1.fn;
identifier bridge;
statement S, S1;
@@
 int fn(
 	struct drm_bridge *bridge
+	, enum drm_bridge_attach_flags flags
 )
 {
 	... when != S
+	if (flags & DRM_BRIDGE_ATTACH_NO_CONNECTOR) {
+		DRM_ERROR("Fix bridge driver to make connector optional!");
+		return -EINVAL;
+	}
+
 	S1
 	...
 }

@ depends on rule1 @
identifier rule1.fn;
identifier bridge, flags;
expression E1, E2, E3;
@@
 int fn(
 	struct drm_bridge *bridge,
 	enum drm_bridge_attach_flags flags
 ) {
 <...
 drm_bridge_attach(E1, E2, E3
+	, flags
 )
 ...>
 }

@@
expression E1, E2, E3;
@@
 drm_bridge_attach(E1, E2, E3
+	, 0
 )

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Tested-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200226112514.12455-10-laurent.pinchart@ideasonboard.com

1 files changed, 17 insertions, 3 deletions

diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
index 018e195c4808..ea2aa5ebae34 100644
--- a/include/drm/drm_bridge.h
+++ b/include/drm/drm_bridge.h
@@ -40,6 +40,17 @@ struct edid;
 struct i2c_adapter;
 
 /**
+ * enum drm_bridge_attach_flags - Flags for &drm_bridge_funcs.attach
+ */
+enum drm_bridge_attach_flags {
+	/**
+	 * @DRM_BRIDGE_ATTACH_NO_CONNECTOR: When this flag is set the bridge
+	 * shall not create a drm_connector.
+	 */
+	DRM_BRIDGE_ATTACH_NO_CONNECTOR = BIT(0),
+};
+
+/**
  * struct drm_bridge_funcs - drm_bridge control functions
  */
 struct drm_bridge_funcs {
@@ -47,7 +58,8 @@ struct drm_bridge_funcs {
 	 * @attach:
 	 *
 	 * This callback is invoked whenever our bridge is being attached to a
-	 * &drm_encoder.
+	 * &drm_encoder. The flags argument tunes the behaviour of the attach
+	 * operation (see DRM_BRIDGE_ATTACH_*).
 	 *
 	 * The @attach callback is optional.
 	 *
@@ -55,7 +67,8 @@ struct drm_bridge_funcs {
 	 *
 	 * Zero on success, error code on failure.
 	 */
-	int (*attach)(struct drm_bridge *bridge);
+	int (*attach)(struct drm_bridge *bridge,
+		      enum drm_bridge_attach_flags flags);
 
 	/**
 	 * @detach:
@@ -757,7 +770,8 @@ void drm_bridge_add(struct drm_bridge *bridge);
 void drm_bridge_remove(struct drm_bridge *bridge);
 struct drm_bridge *of_drm_find_bridge(struct device_node *np);
 int drm_bridge_attach(struct drm_encoder *encoder, struct drm_bridge *bridge,
-		      struct drm_bridge *previous);
+		      struct drm_bridge *previous,
+		      enum drm_bridge_attach_flags flags);
 
 /**
  * drm_bridge_get_next_bridge() - Get the next bridge in the chain